The multi-chip module (MCM) approach for achieving very high densities is of particular interest for high performance, high clock rate digital systems, where signal latency due to interchip interconnect line lengths can prove a major limitation. The increasing speeds of digital integrated circuits can only be realized in increased system speeds if the signal interconnect delays can be correspondingly reduced. Such a reduction of signal propagation delays is achieved principally through the reduction in signal line lengths, such as through dense packaging of bare dies in multi-chip modules. However, due to the substantial areas of IC dies (typically 1 cm. on a side) the interconnect distances between dies in a 2-dimensional (2-D) array can be very substantial when the number of dies is large.
Three-dimensional MCM packaging permits very high density packaging. The advantage of a 3-D array is dramatic: whereas a given die has only 8 nearest neighbors in 2-D, in 3-D (assuming a 1.5 cm. X-Y pitch and a 2.5 mm. Z pitch) 116 dies may be reached with essentially the same signal interconnect length. If these 117 chips had to be spread out horizontally, the delay between chips would represent a significant performance compromise for system clock rates in the 50-100 MHz range, and would virtually preclude operation at rates above 250 MHz. With 3-D packaging, rates of nearly 1 GHz are theoretically possible.
The problem of 3-D architecture is that as IC chips are operated at higher frequencies, their power dissipations tend to increase proportionally. As the chips are packed closely in MCM packaging, the power density levels give rise to severe problems with waste heat.
Eden, in Proceedings of the International Symposium on Hybrid Microelectronics (1991), has pointed out that the key problem in 3-D packaging is achieving the vertical interconnects between boards with high density in a practical, demountable, reworkable fashion, while at the same time getting out of the "cube" (the completed 3-D interconnected system) the heat resulting from the substantial power dissipation from all of the high speed IC's operating in a small volume. If the problems could be solved, a 10 cm. by 10 cm. cube computer could provide all the logic of a 16-processor Cray3.TM. supercomputer. Diamond substrates with metallized vias show promise of providing the solution.
Table I shows the thermal conductivities of some materials that, on the basis of their mechanical properties, could be considered as possible substrates for mounting dies.
TABLE I ______________________________________ Thermal Conductivity, Material W/m-.degree.C. ______________________________________ Diamond: Natural 2000 CVD 500-1600 Beryllia, BeO 223 AlN 70-230 Alumina, 99% 29 GaAs 45 Silicon 149 Kovar .RTM. (FeNiCo) 17 Molybdenum 146 Aluminum 237 Copper 396 Silver 428 Diamond-Epoxy 8.7 Silver-Epoxy 5.8 Polyimide 0.2 ______________________________________
There are no electrical insulators that come within a factor of eight of diamond's thermal conductivity, which is more than four times that of copper. Natural diamond is too expensive at present to be commercially useful, but chemical vapor deposited (CVD) diamond is becoming commercially viable.
The ability to produce high density packing of active and passive components on diamond substrates is greatly limited, however, if there is no technique to pattern and interconnect traces on both sides of the diamond. In other ceramic substrates such as alumina this can be easily accomplished by co-firing the ceramics with metal traces. With CVD diamond this cannot be done because diamond does not sinter at any reasonable temperatures. Thus a technique to fabricate metallized vias in CVD diamond is needed. Ideally the vias would have a high electrical conductivity, the metal would completely fill the hole so that packaging could be hermetically sealed and the metal would adhere well to the diamond.
The present invention provides diamond substrates having tightly adherent metal vias. Said vias are, for the most part, essentially free of voids and flush with the two faces of the diamond.